Our goal was to determine if treatment with insulin-like growth factor (IGF-I) improves nutritional status and promotes growth in undernourished children with cystic fibrosis. We are in the midst of a phase 1-2 double blind crossover study that compares the treatment with IGF-I to placebo in undernourished patients with CF. Malnutrition adversely affects survival and is prevalent among patients with CF who have deteriorating pulmonary function. Nutritional supplementation often requires the use of a gastrostomy or nasogastric tube. However, the feedings are difficult to administer and may further stigmatize children and adolescents with CF. Blood levels of IGF-I, a growth-promoting peptide that is necessary for normal growth, are low in children and adolescents with CF. Since treatment with IGF-I improves nutritional status in undernourished patients, a rationale exists to assess treatment with growth-promoting peptides in those children with CF who are undernourished. Furthermore, IGF-I helps to regulate blood sugar levels. The effect of IGF-I on blood glucose control may be of particular importance since patients with CF have an increased risk to develop diabetes. If IGF-I improves nutritional status, growth, and glucose control, then it will likely enhance functional outcome in undernourished children with CF.